How do pH electrodes work

The bulb is usually fabricated with common soda glass, i.e. glass containing a high concentration of sodium ions. Finally, a small reference electrode, such as an SCE, is positioned beside the bulb. For this reason, the pH electrode ought properly to be called a pH combination electrode, because it is combined with a reference electrode. If the pH electrode does not have an SCE, it is termed a glass electrode (GE). The operation of a glass electrode is identical to that of a combination pH electrode, except that an external reference electrode is required. 

To determine a pH with a pH electrode, the bulb is fully immersed in a solution of unknown acidity. The electrode has fast response because a potential develops rapidly across the layer of glass 

Empirical means found from experiment, rather than from theory. 

Thin-walled glass bulb Silver wire 

The magnitude of the potential developing across the glass depends on the difference between the concentration of acid inside the bulb (which we know) and the concentration of the acid outside the bulb (the analyte, whose pH is to be determined). In fact, the emf generated across the glass depends in a linear fashion on the pH of the analyte solution provided that the internal pH does not alter, which is why we buffer it. This pH dependence shows why a pH meter is really just a pre-calibrated voltmeter, which converts the measured emf into a pH. It uses the following formula  

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